Ethene can be polymerized in the gas phase or in the liquid phase. Both processes have advantages and also disadvantages.
The gas phase polymerization has the advantage that only small amounts of a dispersion medium, if any, are required. However, it has the disadvantage that maintaining a constant polymerization temperature over the entire reaction space is substantially more difficult and that part of the polyethene obtained is produced in the form of agglomerates. Furthermore, the polymerization partially takes place directly at the walls of the apparatus. This material caked to the wall partially detaches from it during the polymerization and, thus, passes in the form of lumps and small flat blocks into the normal polyethene powder. These polyethene agglomerates cause blockages of the apparatus and contaminate the polyethene powder; in the finished article, they cause disclolorations and specks. Furthermore, in such gas phase polymerizations, uniform distribution of the mixed catalyst is difficult. Equally deleteriously, the control of molecular weight, the manufacture of polyethene of low molecular weight and, especially, the manufacture of polyethene of high density are all very difficult or impossible. Furthermore, the polyethene obtained in the gas phase polymerization has a relatively low bulk density.
It is true that polymerization in the liquid phase does not suffer from such disadvantages, but, on the other hand, it requires use of large amounts of dispersion media. Furthermore, the liquid phase polymerization can only be carried out for as long as the suspension can be stirred. When ethene is added as a gas, the rate of polymerization in general decreases continuously as the stirrability of the polyethene dispersion decreases. Finally, when the stirrability is very low, polymerization is no longer detectable. This termination of polymerization in general occurs at solids contents of about 200 to 300 g of polyethene/liter of suspension, depending on the bulk density of the polyethene. For these reasons, polymerization in the liquid phase can in general only be carried out to solids contents of about 200 to 300 g/l of suspension. This not only requires the use of large amounts of dispersion media and catalysts, and correspondingly high costs for these, but also entails high contents of catalysts in the polyethene obtained. Therefore, an expensive working-up is required to substantially remove these catalysts.